In recent years, communication services using a long term evolution (LTE) system are spreading nationwide. In communication between a base station and a terminal in the LTE system, a number concerning the order of a packet transmitted and received between the base station and the terminal is managed in a PDCP (Packet Data Convergence Protocol) layer. This number is specified by an HFN (Hyper Frame Number) and a PDCP-SN (Sequence Number) (NPL 1).
The HFN is a common number held by the base station and the terminal. The header of a packet is compressed using the HFN. Hence, the HFN needs to match between the base station and the terminal. On the other hand, the PDCP-SN is a number notified by a device on the transmitting side, and its value is incremented one by one on a packet basis. When the PDCP-SN reaches the maximum value, the devices on the transmitting and receiving sides increment the HFN by one.
That is, assume that the maximum value of the PDCP-SN is 127. If 128 packets with PDCP-SNs “0” to “127” are transmitted/received, the HFN is incremented by one. On the other hand, after reaching the maximum value, the PDCP-SN is reset to 0. If the PDCP-SN of a received packet is larger than the PDCP-SN of a previously received packet, the device on the receiving side does not increment the HFN. If the PDCP-SN of the received packet is equal to or smaller than the PDCP-SN of the previously received packet, the device increments the HFN. In this way, the device on the transmitting side and the device on the receiving side manage the order of a packet using the HFN and PDCP-SN, and compress or decode the header of the packet using the HFN of them.